1. Field of the Invention
The present invention relates to a device and a method for selecting codes, and can be applied for a code selection device used to select spreading code (hereinafter referred to as “code”) in order to allocate to a communication apparatus employing, for example, a code division multiple access (CDMA) system, a new communication channel having a preferable correlation property.
2. Related Background Art
Conventionally, a time division multiple access (TDMA) system is mainly employed as an access system for an optical access network, for example. According to the TDMA system, however, when a plurality of channels are multiplexed to a single optical fiber, the period of time it can be used by an individual user is fixed, and an unused period, during which communication by a designated, specified user is not performed, can not be utilized by another user. Therefore, the band available for each user across the network can not be utilized to the maximum. To resolve this problem, a CDMA system has been proposed.
An example method for providing a CDMA system via an the optical access network is disclosed, for example, in reference: “COMMUNICATION USING SPREAD SPECTRUM METHODS OVER OPTICAL FIBERS”, WO9907087. According to this method, a transmission side modulates an optical carrier that has been coded (spectrum spreading) by using a spreading code sequence (hereinafter referred to as “code”) in an electric region, and a reception side directly detects an optical modulated signal and decodes (spectrum despreading) the signal in an electric region.
With this method proposed in the reference, while an increase in the data transmission rate for one communication channel is limited, the standard parts used for a conventional CDMA system, such as a mobile communication system, can be employed. Therefore, the CDMA system can be provided comparatively easily.
In addition, since wavelength control and filtering are not required in an optical region, it is possible to construct the system at a low cost.
For a network using the CDMA system in addition to the optical access network in the above reference, the following two methods are available for the allocation of communication channels.
The first method is a method for allocating, to the transmitter of each terminal, a fixed code sequence (spreading code sequence) that is already known in a network. The second method is a method whereby, instead of allocating a fixed code to the transmitter of each terminal, a code sequence is transmitted when a connection is established by using a control line, and a code is allocated each time communication is initiated. Example code sequences used for this method are the M sequence, the Gold sequence and the Kasami sequence.
For a network that uses the CDMA system, it is known that the number of channels is extremely reduced when a code sequence is used, while taking into account the phase difference between different channels, that is a complete orthogonal sequence.
Therefore, generally, systems tend to be constructed that use incomplete orthogonal code sequences. As a result, when different channels employ a communication path at the same time, due to the non-orthogonality of the codes that are employed, an interference occurs between the communication channels, and the number of occupation channels is limited.
The magnitude of the inter-channel interference depends on the phase difference between the communication channels and the correlation property of the code that is currently being employed. Thus, in order to increase the number of occupation channels, it is preferable that codes having a preferable correlation property be selected and allocated.
For the above described two communication channel allocation methods, code sequences are employed for which the correlation properties are regarded as being preferable; however, a method is not described whereby to select and allocate one of several types of code sequences that are to be used for the entire network.
Therefore, a demand exists for the provision of a method whereby, in accordance with the usage condition of a network, a code sequence having a preferable correlation property can be selected, from among a number of code sequences that are generated, and allocated to a communication channel.